A Digital Subscriber Line (DSL) connection is a connection that allows for the provision of digital communication over existing twisted copper pair subscriber lines. The term DSL is a collective term to cover a number of variations on the DSL technology, including ADSL (“Asymmetric” DSL), SDSL (“Symmetric” DSL), ADSL2+(a technique that extends the capability of basic ADSL by doubling the number of downstream channels), VDSL (Very-high-bit-rate DSL), VDSL2 (an improved version of VDSL), and others, including, in particular, “G.fast”, which will be discussed later.
In general, a DSL connection comprises a copper subscriber line extending between two DSL modems. A first DSL modem is typically located at the customer's premises, and the second modem may be located at the local exchange (known as the ‘central office’ in US terminology), in a street cabinet, or distribution point (DP). (NB The acronym “DP” is sometimes also used to refer to a ‘drop point’, and the distribution point may in fact be a drop point, but in general, where the acronym “DP” is used here, it will be used to refer to the term “distribution point”, whether this is a drop point or otherwise).
Typically, the local exchange, street cabinet or distribution point includes a DSL Access Multiplexer, DSLAM (a form of aggregation transceiver device) comprising several DSL modems (one for each subscriber line). The DSLAM (at the exchange, cabinet or distribution point) connects the first DSL modem at the customer's premises to the Core Network, typically over a faster optical fibre connection, and a Network Management System (NMS).
FIG. 1 illustrates a hierarchical relationship between the local exchange (“Exchange”) (from which telephony is generally also handled), a street cabinet (“Cab”), a distribution point (“DP”) and three customer premises (“CP”). In general, it should be noted that there may—and generally will—be several street cabinets and several distribution points for each exchange, and many more than three customer premises, but the entities shown in FIG. 1 are intended to illustrate this hierarchical relationship, and to illustrate different ways in which customer premises may be connected to the core network. In this example, the local exchange DSLAM can be thought of as being at Level 1 of the hierarchy and connects a first set of customers' DSL modems to a Core Network. The street cabinet DSLAM can be thought of as being at Level 2 of the hierarchy and connects a second set of customers' DSL modems to the Core Network through the exchange. The distribution point DSLAM can be thought of as being at Level 3 of the hierarchy and connects a third set of customers' DSL modems to the Core Network through the exchange. All levels of the DSL hierarchy may transmit data over the same frequency domain.
It is well-known that transmissions on one subscriber line may cause interference on another subscriber line. This is known as ‘crosstalk’. Furthermore, it is common for the different sets of customers' DSL modems to be bundled together (for example, a subscriber line between one of the first set of customers' DSL modems and the local exchange may be bundled together with a subscriber line between one of the third set of customers' DSL modems and the distribution point, as shown in FIG. 1). However, subscriber lines between higher-tier DSLAMs and their customers' DSL modems tend to be significantly longer than subscriber lines for lower-tier DSLAMs, such that a signal on the longer subscriber line is significantly attenuated by the point at which it is bundled together with the subscriber lines for lower-tier DSLAMs. Thus, full power transmissions by lower-tier DSLAMs can often be the cause of high levels of crosstalk on higher-tier subscriber lines using the same frequencies.
This general problem has been addressed by a technique called the Access Network Frequency Plan (ANFP). The ANFP preserves resources for the exchange by defining the Power Spectral Density (PSD) of transmissions by lower-tier DSLAMs. For example, the PSD for the cabinet's DSLAM is generally shaped such that the power level at any frequency that may also be being used by the exchange DSLAM is reduced. This generally decreases the chance of crosstalk on the DSL connections between the exchange and the customer. This technique may also be applied to the distribution point's DSLAM (such that the PSD is shaped according to the resources being used by the street cabinet and the exchange).
In order to provide customers with higher data rates, the location of the network-side DSLAM for a particular customer's line is generally being moved closer to the customer (i.e. from the exchange towards the distribution point). Thus, copper pair connections between the local exchange and street cabinets and distribution points are being replaced with optical fibre, such that the length of the copper subscriber line (with its inherent data rate limitations) is reduced. There is therefore a trend in providing a greater number of DSLAMs in street cabinets and distribution points.
A problem with the ANFP identified by the present inventors is that while the ANFP generally reduces the chance of crosstalk on higher-tier DSL connections, it generally also reduces the capacity of lower-tier DSLAMs when there is overlap in the frequencies used both by lower and higher tier DSLAMs. As DSLAMs are located closer to customers, the ANFP technique will become increasingly more inefficient in terms of the total capacity of the network.
Prior art techniques generally involve the Network Management System at the exchange determining the resource allocations for subscriber lines offline. This is generally a manual and time-consuming process for the Network Operator in question. Once the resources have been allocated, the Network Management System generally sends an allocation message to the DSLAM in question, which then implements the resource allocation to the line or lines in question at the next retrain. Such retrains can be frustrating for end-users as they generally result in loss of service for a period of time which may be several minutes. Accordingly, the Network Operator must either wait for the line to retrain, or force a retrain on the line at an appropriate time (e.g. in the middle of the night). Either way, there is generally a significant delay before the resource allocations for all lines in the network are implemented.
It is well known that the performance of DSL technologies is directly related to the ratio of the signal to noise (the SNR) at the receiver. There are many variables that affect the SNR, including the length/loss of the circuit (i.e. the copper pair) and the noise coupled into the circuit, but with DSL technologies, crosstalk (discussed above) is one of the major sources of noise into a circuit.
Looking into this in more detail, the twisted copper pair carrying a DSL circuit for a particular customer's connection rarely exists in isolation, and is normally bundled up into a cable with many other pairs (those for other customers' connections). There will generally be crosstalk (i.e. coupling between the respective pairs in a cable), which results in some of the signal transmitted on one pair coupling into other pairs. This means that unwanted signal from other circuits transmitters is received at each receiver as noise.
Crosstalk noise causes significant bit rate reduction for DSL services. The impact is generally worst when transmitters aren't collocated; the crosstalk from close transmitters (i.e. transmitters close to the customer premises) into lines where the transmitter is further away can result in very low SNRs, and hence low or decreased bit rates.
Crosstalk cancellation technology (often referred to as “vectoring”) has recently reached maturity for VDSL2. This effectively mitigates the impact of crosstalk, but only for circuits connected to the same DSLAM where the cancellation algorithms are run. “Alien” crosstalk (i.e. crosstalk from circuits connected to other DSLAMs) can remove much of the gain from vectoring.
The present inventors have realised that this may be a particular issue in relation to “G.fast” (referred to briefly earlier), which is a further DSL standard under development (in early 2014) by the ITU-T, and which relates to a DSL technology capable of enabling faster aggregate uplink and downlink data rates than are possible with VDSL2. G.fast data rates are currently expected to be up to 1 Gbit/s over existing copper telephone wires shorter than 250 m. The name “G.fast” comprises an acronym for “Fast Access to Subscriber Terminals” together with the letter “G” from the ITU-T “G series” of recommendations (ITU-T G.9700 and G.9701) concerned.
It will be noted that while G.fast is a further development of technology used in VDSL2, it is optimised for shorter distances such as those from distribution points to customer premises (i.e. up to around 250 m)—it is not intended to be used as a replacement for VDSL2 over longer distances.
In G.fast (as in VDSL2 and most ADSL variants), data is modulated using discrete multi-tone (DMT) modulation. G.fast modulates up to 12 bits per DMT frequency carrier (compared with 15 in VDSL2). Unlike ADSL2 and VDSL2, G.fast uses time-division duplexing (ADSL2 and VDSL2 use frequency-division duplexing).
To avoid the potential impact on vectored VDSL2 deployed at the cabinet from G.fast deployed at the distribution point, the initial assumption has been to use statically non-overlapped spectrum. For example, VDSL2 in the UK is limited to 17 MHz. Due to the slow roll-off of the G.fast PSD, the transmitted power only becomes insignificant 5 MHz below the lowest used frequency. As a result, G.fast may be deployed with a fixed lower frequency of 23 MHz, which sacrifices at least 200 Mb/s when compared to a lower frequency of 2 MHz. This is because in order to maintain spectral compatibility with VDSL from the cabinet (i.e. a maximum frequency of 17.667 MHz), a static method would need to start at 23 MHz. This would prevent the use of carriers between 2.2 MHz (the top of ADSL2+) and 23 MHz, with a theoretical capacity of approximately 200 Mbps.
In the UK, the ANFP aims to balance harm using static spectrum management PSDs which are selected at different locations based on loop records. This technique relies on accurate records and must be set up with the worst case assumption of few circuits, high crosstalk. It doesn't account for the actual DSL performance and which customers are enabled, but despite being conservative would recover some of the >200 Mb/s in some locations.
Other techniques involve connecting complex Dynamic Spectrum Management (DSM) systems to both the short and the long systems which operate close to real time using optimal spectrum balancing, iterative “water filling” or similar algorithms to optimise the PSDs with the aim of achieving the lowest starting frequency individually on each G.fast line. Such techniques are complex, require large quantities of data, and are difficult to implement between multiple Network Operators for commercial, practical and regulatory reasons.
It is therefore desirable to alleviate some or all of the above problems.
Referring now to specific prior art techniques and disclosures, United States patent application US2008/0192813 (“Stolle et al”) relates to techniques for adjusting transmit power spectra of both subscriber devices and central transmit/receive devices of a communications network. The specification of this patent application refers to a feature called “Downstream Power Back Off” (DPBO). This uses attenuation (the electrical losses of cables) to estimate the received signal from the exchange and to set the power transmitted from a cabinet at this level so the cabinet transmissions cause no more harm than other additional exchange services would.
European patent application EP1370057 (“Alcatel”) relates to techniques for the deployment of ADSL services. It suggests using records of line lengths to estimate frequencies which are likely to be being used.
US2012275576 (“Wei”) relates to a downstream power back-off technique for digital subscriber lines, and in particular to a method comprising determining power spectrum density (PSD) profiles for a plurality of cabinet-deployed lines by jointly and iteratively determining cut-off frequencies based on crosstalk coupling parameters among the lines, the cut-off frequencies and PSD profiles being in one-to-one correspondence, each profile comprising a reduced PSD portion below the cut-off frequency of the profile and a maximum PSD portion above the cut-off frequency. The technique disclosed requires knowledge of the PSD transmitted by each other line together with knowledge of the crosstalk coupling factors concerned.
WO10018562 (“ECI Telecom”) relates to noise mitigation in xDSL lines based on conventional downstream power back-off (DPBO), upstream power back-off (UPBO) and Virtual Noise (VN) techniques. It aims to enable dynamic, more accurate determination of parameters required for management of transmission in xDSL channels.
US2010027601 (“Fang”) relates to techniques for controlling DSL transmission power, involving determining a group to which a line belongs, and selecting a representative line from each group to form a line model; obtaining a crosstalk model of the line model; obtaining Transmit Power Spectrum Density (TxPSD) of each representative line in the line model according to the crosstalk model, and converting the TxPSD into a spectrum control parameter; and enhancing the line rate according to the spectrum control parameter.